The present invention relates, in general, to the field of radio frequency ("RF") identification ("ID") systems. More particularly, the present invention relates to a passive RF transponder incorporating a non-volatile memory element which is powered by inductive coupling to a proximately located RF controller. A communication system and method utilizes the controller to frequency shift key ("FSK") the powering RF signal to cause the transponder to either read data from or write data to the non-volatile memory and transmit the same back to the controller utilizing phase shift keying ("PSK") modulation in response to commands from the controller. The communication system may operate in either full duplex or half duplex modes with the controller causing the transponder to simultaneously write data to its non-volatile memory array as the contents are read out and transmitted to the controller by complementing selected bits thereof.
Many potential applications exist for RF/ID devices or cards that may be readily carried by a user and have their memory contents read when placed near a reader/controller. A still greater number of applications can be found for a true card sized RF transponder to which data may be written as well.
As a practical matter, the card form factor generally precludes the use of a continuous on-board power source such as a battery, which in turn eliminates the possibility of data retention through the use of conventional volatile semiconductor storage devices such as dynamic random access memory ("DRAM") or battery backup static random access memory ("BBSRAM"). Furthermore, the write times and power requirements for conventional non-volatile memories such as electrically erasable programmable read only memory ("EEPROM") and non-volatile random access memory ("NOVRAM") render their use in a passive transponder extremely problematic if not unworkable.
An alternative non-volatile memory technology for retention of data in a passive RF transponder includes the use of a ferroelectric random access memory ("FRAM.RTM.") integrated circuit memory array utilizing a proprietary lead-zirconate-titanate ("PZT") ceramic thin film available from Ramtron International Corporation, Colorado Springs, Colo. Through the use of FRAM technology, it is possible to obtain sufficient power to write to the memory array and power associated logic from a relatively weak external RF field. A proposed RF/ID tag utilizing FRAM technology is described in the literature relating to the "RTx 0801 Ramtag".COPYRGT. 1990 Ramtron Corporation. The engineering prototype therein described, postulated a passive RF/ID tag which could be powered in an FSK modulated RF field varying between 500 KHz (logic one) and 400 KHz (logic zero). The tag itself was designed to power up in a "read" mode and respond to the powering "reader" also using an FSK modulated signal of 50 KHz (logic one) and 40 KHz (logic zero). As a proposed communications protocol, it was suggested that there be a synchronization ("SYNC") field of 1.5 bit time of logic one followed by 1.5 bit time of logic zero thereafter followed by one of four possible commands: 00 for "read"; 01 for "write" enable; 10 for "write" and 11 for "write" disable. As previously described, the tag was initialized upon power up in a "read" mode of operation and did not identify itself to the reader as to its pre-programmed identity or configuration. In addition, through the proposed use of separate write enable and disable commands, a less than efficient communications protocol is defined that does not accord full duplex operation. The card and reader, as proposed, could also only read or write the entire memory contents at one time and could not address selected words or rows of memory.